You will never be "infected with kde 4 updates", even when support ends you will be able to continue using the system (but slightly less securely). Elyssa is a Long Term Support (LTS) release, meaning it will be supported for 3 years in total. According to http://www.linuxmint.com/download_lts.php this is currently equivalent to April 2011

If you have a question that has been answered and solved, then please edit your original post and put a [SOLVED] at the end of your subject headerHint - use a google search including the search term site:forums.linuxmint.com